1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell employing a so-called olivinic lithium phosphorus oxide as a cathode active material.
2. Description of Related Art
Nowadays, in keeping up with the recent marked progress in the electronic equipment, researches into re-chargeable secondary cells, as power sources usable conveniently and economically for prolonged time, are underway. Representative of the secondary cells are lead accumulators, alkali accumulators and non-aqueous electrolyte secondary cells.
Of the above secondary cells, lithium ion secondary cells, as non-aqueous electrolyte secondary cells, have such merits as high output and high energy density.
The lithium ion secondary cells are made up of a cathode and an anode, including active materials capable of reversibly doping/undoping lithium ions, and a non-aqueous electrolyte. The charging reaction of the lithium ion secondary cell proceeds as lithium ions are deintercalated into an electrolyte solution at the cathode and are intercalated into the anode active material. In discharging, reaction opposite to that of the charging reaction proceeds, such that lithium ions are interecalated at the cathode. That is, charging/discharging is repeated as the reaction of entrance/exiting of lithium ions from the cathode into the anode active material and from the anode active material occurs repeatedly.
As the cathode active material of the lithium ion secondary cell, LiCoO2, LiNiO2 or LiMn2O4 is used because these materials have a high energy density and a high cell voltage.
In a non-aqueous electrolyte secondary cell, employing a lithium composite oxide and a carbonaceous material as a cathode active material and as an anode active material, respectively, leakage of the electrolyte solution or cell destruction is produced due to rise in the internal pressure caused by gases generated by the reaction between the cathode and the electrolyte solution during charging/discharging.
For combating these inconveniences, it is necessary to provide a void in the cell container. However, if such a void is to be provided, the amount of the active material that can be accommodated in the container needs to be diminished, thus lowering the apparent volumetric energy density of the cell.